A liquid crystal display is widely utilized in monitors of personal computers and mobile appliances and applications for TV because of various advantages that downsizing and thinning can be achieved at low voltage and low electric power consumption. In such a liquid crystal display, various modes are proposed depending upon an alignment state of liquid crystal molecules within a liquid crystal cell. A TN mode taking a twisted alignment state of about 90° toward an upper substrate from a lower substrate of the liquid crystal cell has hitherto been the mainstream.
In general, a liquid crystal display is configured of an optical compensation sheet and a polarizer. The optical compensation sheet is used for the purpose of overcoming image coloration or enlarging a viewing angle, and a stretched birefringent film or a film having a liquid crystal coated on a transparent film is used. For example, Japanese Patent No. 2587398 (corresponding to U.S. Pat. No. 5,583,679) discloses a technology for applying an optical compensation sheet in which a discotic liquid crystal is coated, oriented and immobilized on a triacetyl cellulose film to a liquid crystal cell of TN mode and enlarging a viewing angle.
However, in a liquid crystal display for television applications in which it is supposed that a person looks from various angles in a large-sized screen, demands for the viewing angle dependency are severe. Even by the foregoing method, these demands cannot be satisfied. For that reason, liquid crystal displays different from the TN mode, for example, an IPS (in-plane switching) mode, an OCB (optically compensatory bend) mode and a VA (vertically aligned) mode are studied. In particular, the VA mode is watched as a liquid crystal display for TV because it is high in contrast and relatively high in manufacture yield.
Now, a cellulose ester film is characterized in that its optical isotropy is high (low in a retardation value) as compared with other polymer films. Accordingly, it is general that the cellulose ester film is used for applications in which optical isotropy is required, for example, a polarizing plate.
On the other hand, an optical compensation sheet (phase difference film) of the liquid crystal display is inversely required to have optical anisotropy (high retardation value). In particular, it is considered that an optical compensation sheet for VA mode is required to have a front retardation (Re) of from 30 to 200 nm and a retardation (Rth) in a film direction of from 70 to 400 nm. Accordingly, it was general that a synthetic polymer film having a high retardation value, such as polycarbonate films and polysulfone films, is used as the optical compensation sheet.
In the foregoing technical field of optical materials, it was a general rule that when the polymer film is required to have optical anisotropy (high retardation value), a synthetic polymer film is used, whereas when the polymer film is required to have optical isotropy (low retardation value), a cellulose ester film is used.
European Patent No. 0911656A2 proposes a cellulose ester film having a high retardation value, which overturns the conventional general rule and is able to be used for applications requiring optical anisotropy. In this proposal, in order to realize a high retardation value in cellulose triacetate, a stretching treatment is carried out by adding an aromatic compound having at least two aromatic rings, especially a compound having a 1,3,5-triazine ring. In general, it is known that cellulose triacetate is a hardly stretchable, high-molecular weight raw material and is difficult to make the birefringence large. However, by simultaneously orienting the additive in the stretching treatment, it becomes possible to make the birefringence large, thereby realizing a high retardation value. Since this film is also able to work as a passivation film for polarizing plate, there is brought an advantage that an inexpensive and thin liquid crystal display can be provided.
JP-A-2002-71957 discloses an optical film containing a cellulose ester which has an acyl group having from 2 to 4 carbon atoms as a substituent and which, when a degree of substitution of an acetyl group is defined as A and a degree of substitution of a propionyl group or a butyryl group is defined as B, is simultaneously satisfied with relationships of {2.0≦(A+B)≦3.0} and {A<2.4}, wherein a refractive index Nx in a slow axis direction and a refractive index Ny in a fast axis direction at a wavelength of 590 nm are satisfied with a relationship of {0.0005≦(Nx−Ny)≦0.0050}.
JP-A-2003-270442 (corresponding to US2003/0156235A1) discloses a polarizing plate to be used for a VA mode liquid crystal display having a polarizer and an optically biaxial mixed fatty acid cellulose ester film, wherein the optically biaxial mixed fatty acid cellulose ester film is arranged between a liquid crystal cell and the polarizer.